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Subject: Re: [virtio-dev] Re: [Qemu-devel] [PATCH v3 1/2] virtio-net: enable configurable tx queue size

On 06/26/2017 04:05 PM, Jason Wang wrote:

On 2017年06月26日 12:55, Wei Wang wrote:
On 06/26/2017 11:18 AM, Jason Wang wrote:

On 2017年06月23日 10:32, Wei Wang wrote:
This patch enables the virtio-net tx queue size to be configurable
between 256 (the default queue size) and 1024 by the user when the
vhost-user backend is used.

Currently, the maximum tx queue size for other backends is 512 due
to the following limitations:
- QEMU backend: the QEMU backend implementation in some cases may
send 1024+1 iovs to writev.
- Vhost_net backend: there are possibilities that the guest sends
a vring_desc of memory which corsses a MemoryRegion thereby
generating more than 1024 iovs after translattion from guest-physical
address in the backend.

Signed-off-by: Wei Wang <wei.w.wang@intel.com>
hw/net/virtio-net.c | 45 +++++++++++++++++++++++++++++++++---------
  include/hw/virtio/virtio-net.h |  1 +
  2 files changed, 37 insertions(+), 9 deletions(-)

diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
index 91eddaf..d13ca60 100644
--- a/hw/net/virtio-net.c
+++ b/hw/net/virtio-net.c
@@ -34,8 +34,11 @@
    /* previously fixed value */
+    /*
+ * Currently, backends other than vhost-user don't support 1024 queue
+     * size.
+     */
+    if (n->net_conf.tx_queue_size == VIRTQUEUE_MAX_SIZE &&
+        nc->peer->info->type != NET_CLIENT_DRIVER_VHOST_USER) {
+        n->net_conf.tx_queue_size = VIRTIO_NET_TX_QUEUE_DEFAULT_SIZE;

Do we really want assume all vhost-user backend support 1024 queue size?

 Do you know any vhost-user backend implementation that doesn't support
1024 tx queue size?

I don't but the issue is vhost-user uditis an open protocol, there could be even closed source implementation on this. So we can't audit all kinds of backends.

This is the discussion left before. Here is my thought:

Until now, nobody is officially using 1024 tx queue size, including the unknown
vhost-user backend implementation. So, applying this patch won't affect any
already deployed products.

If someday, people whose products are based on the unknown vhost-user
implementation change to try with 1024 tx queue size and apply this patch,
and find 1024 doesn't work for them, they can simply set the value to 512 (which
is better than the 256 size they were using) or apply the 2nd patch entitled
"VIRTIO_F_MAX_CHAIN_SIZE" to use 1024 queue size.

I didn't see any big issue here. Would you see any issues?

@Michael, what is your opinion?

+    }
+    for (i = 0; i < n->max_queues; i++) {
+        virtio_net_add_queue(n, i);
+    }

Any reason to move virtio_net_add_queue() here?

Please check the whole init steps. It was moved here (after qemu_new_nic())
to make sure nc->peer is not NULL.

I don't get here, can't you get peer just from nic_conf.peers?

Correct, nic_conf.peers also works. Thanks.


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